In comparison with cathode ray tube (CRT) displays, poor viewing angle performance is a major shortcoming in conventional LCD displays. In order to solve this problem, In-Plane Switching (IPS) mode LCDs and FFS mode LCDs have been developed. A common electrode and a pixel electrode of the IPS mode LCD are disposed on a same substrate, and a horizontal electric field, which is formed between the common electrode and the pixel electrode, is utilize to twist liquid crystal molecules in a plane. The viewing angle performance of the IPS mode LCD can be significantly improved, but an aperture ratio thereof is low since the common electrode and the pixel electrode are disposed on the same substrate.
The disposal scheme of the electrodes in the IPS mode LCD is improved in the FFS technology, the common electrode of an opaque metal in the IPS mode LCD is replaced by a transparent common electrode, which is made as board-like, for increasing a transmittance to overcome the shortcoming of the low aperture ratio. Moreover, unlike that positive and negative electrodes of the IPS mode LCD are arranged apart, positive and negative electrodes of the FFS mode LCD are arranged overlapped with an insulative layer sandwiched therebetween so that widths of the electrodes and an interval therebetween can be greatly reduced. This design allows the distribution of the electric field to be more intensive.
Referring to FIG. 1 and FIG. 2, FIG. 1 is a top view schematically illustrating a pixel structure of an FFS mode LCD in prior art, and FIG. 2 is a schematic cross-sectional diagram along a line A-A′ in FIG. 1. The pixel structure of FFS mode LCD 10 includes a first substrate 11, a second substrate 12, and a liquid crystal layer 13 is sandwiched between the first substrate 11 and the second substrate 12. A common electrode 14, an insulative layer 15, a pixel electrode 16, and a first alignment film 17 are sequentially laminated on a side of the first substrate 11 which is adjacent to the liquid crystal layer 13. A color layer 18, a protective layer (overcoating) 19, and a second alignment film 20 are sequentially disposed on a side of the second substrate 12 which is adjacent to the liquid crystal layer 13. The protective layer 19 herein is utilized to avoid a height difference being generated between a black matrix (BM) 182 and a chromatic photoresist 184 in the color layer 18, that is, to planarize the whole surface thereof for facilitating the second alignment film 20 to be coated smoothly.
As shown in FIG. 1, a plurality of scan lines 111, data lines 112 and storage capacitor electrodes 113 are disposed on the first substrate 11. The scan lines 111 insulatively intersect the data lines 112 to define a plurality of pixel units (not shown). Thin film transistors (TFTs) 110 are disposed in the intersection regions of the scan lines 111 and the data lines 112. The pixel electrode 16 overlaps the common electrode 14 in each pixel unit, the pixel electrode 16 is a comb-like structure, and the common electrode 14 is a board-like structure. When no voltage is applied to the pixel electrode 16, the liquid crystal molecules are arranged along an alignment direction, that is, a direction parallel to the scan lines 111.
As shown in FIG. 2, when a voltage is applied to the pixel electrode 16, a fringe field 130 is generated between the pixel electrode 16 and the common electrode 14, and the liquid crystal molecules are twisted in a plane due to the action of the fringe field. The liquid crystal molecules are twisted to a direction that is perpendicular to slits of the comb-like structure of the pixel electrode 16 as shown in FIG. 1, thereby controlling backlight outgoing.
However, when there is no voltage applied to the pixel electrode 16, the backlight is unable to pass through the liquid crystal layer 13 in theory. Actually, the first alignment film 17, which covers over the TFT 110, the storage capacitor electrode 113, as well as edges of the pixel electrode 16, is not even and has some height differences thereon, such that the alignment direction of the liquid crystal molecules on those regions is not completely horizontal. Therefore, there is light leakage occurring in those regions. In addition, an electrical field in those regions is not the same as the fringe field 130 shown in FIG. 2, but is a disordered electrical field which can cause incorrect image display.
In order to solve said drawbacks in the prior art, the opaque black matrix 182 is disposed in the color layer 18 on the second substrate 12, thereby shielding the light leakage in those regions. Moreover, the light leakage can be shielded by increasing areas of metal material on the first substrate 11. However, every one of the above-mentioned methods has a shortcoming that the aperture ratio of the panel is reduced.